nd I'll spot the hole. Quick, man!"

"Okay!"

Thomas' long legs shot him out of the headquarters tent. Just beyond
the entrance flap was one of the two gyrocopters used for flying
within the Dome. He leaped into the cockpit and drove home the
starter-piston. The flier buzzed straight up, shooting for the misted
roof.

* * * * *

The Earthman fought to steady his craft against the hurricane wind,
while his gray eyes swept the three-mile circle of the vault's base.
He paled as he noted the fierce speed with which the white smoke-jets
were being torn from the pipe provided for just such emergencies. His
glance followed the terrific rush of the vapor. Big as a man's head, a
hole glared high up on the Dome's inner surface. Feathered wisps of
tell-tale vapor whisked through it at blurring speed.

"God, but the air's going fast," Darl groaned. The accident he had
feared through all the months he had captained Earth's outpost on
Mercury had come at last. The Dome's shell was pierced! A half-mile
high, a mile across its circling base, the great inverted bowl was all
that made it possible for man to defy the white hell of Mercury's
surface. Outside was an airless vacuum, a waste quivering under the
heat of a sun thrice the size it appears from Earth. The silvered
exterior of the hemisphere shot back the terrific blaze; its
quartz-covered network of latticed steel inclosed the air that all
beings need to sustain life.

Darl tugged desperately at the control-stick, thrust the throttle over
full measure. A little more of this swift outrush and the precious air
would be gone. He caught a glimpse of the Dome floor beneath him and
the shaft-door that gave entrance to the mine below. Down there, in
underground tunnels whose steel-armored end-walls continued the Dome's
protection below the surface, a horde of friendly Venusians were
laboring. If the leak were not stopped in a few minutes that shaft
door would blow in, and the mine air would whisk through the hole in
its turn. Only the Dome would remain, a vast, rounded sepulcher,
hiding beneath its curve the dead bodies of three Earthmen and the
silent forms of their Venusian charges.

* * * * *

Darl's great chest labored as he strove to reach the danger spot.
Invisible fingers seemed to be clamped about his throat. His eyes
blurred. The gyrocopter was sluggish, dipped alarmingly when it should
have darted, arrow-l